In the context of the present invention, fouling refers to the deposition or accumulation of unwanted material on a solid surface, most often in an aquatic environment. Fouling can involve living organisms (referred to as “biofouling”) or non-living substances that are inorganic or organic in nature. The solid surface is intended to perform some function and fouling can impede or interfere with that function. Fouling may also give rise to environmental or health issues. For these reasons, fouling is preferably reduced or avoided all together.
In particular, biofouling is a highly diverse problem which affects all manner of submerged, man-made surfaces. Biofouling tends to occur in all aqueous environments, but is particularly prevalent in marine environments. Marine biofouling has enormous economic impact on shipping, offshore oil and gas rigs, power and desalination plants and aquaculture.
Historically, one of the earliest preventive measures taken against marine biofouling was the sheathing (or cladding) of wooden vessels with copper and copper alloys. Copper and its alloys are particularly effective in providing protection against the majority of biofouling organisms. However, with the advent of iron-hulled ships, galvanic corrosion between copper and iron became an issue.
Biocide-loaded paint films became the standard practice for protection of ship hulls, and have continued to be so to this day. During the 20th century, paint formulations became reliant on highly toxic biocides, most notably tributyltin (TBT). However, the use of TBT in antifouling paints on ships has now been prohibited by the International Maritime Organization (IMO) due to concerns about the ecological consequences of its widespread use. In any case, a disadvantage of relying on paint systems is the need to periodically recoat. Furthermore, outage for recoating can have significant cost implications for commercial vessels.
On the other hand, the use of copper and copper alloy sheathing has been demonstrated to exhibit long term antifouling activity, and continues to be used in certain applications, such as pipe work and the legs of oil and gas platforms. Copper-nickel alloys are often preferred over pure copper because they are more resistant to erosion. Other metals such as zinc are also known to provide biofouling protection, although the effect is generally more short-lived than for copper. However, the sheathing/cladding approach is not without problems. Typically, the metal/metal alloy is applied in the form of tiles and problems can arise when it comes to bonding tiles to a hull surface, especially in relation to more complex-shaped surfaces. In this case, the tiles may also need to be specially tailored.
This problem may be overcome by the kind of approach taught in U.S. Pat. No. 4,751,113. This patent describes a method of applying an antifouling coating to a marine surface which involves grit blasting the surface, coating with an adhesion and seal layer and depositing a continuous layer of metal or metal alloy by thermal spraying high velocity molten or semi-molten metal particles. However, this approach is somewhat involved and, as will be apparent, thermal spraying is not applicable to all materials used in marine service.
In U.S. Pat. No. 5,284,682 a coating is applied to the hull of a boat using a two part epoxy-based thermosetting adhesive coating. Copper or copper alloy particles are incorporated into the adhesive by premixing the particles into an epoxy prepolymer or polyamide hardener liquid component prior to application to the hull. Once applied the adhesive coating is cured at ambient temperature. Oblong particles have been found to beneficially concentrate near the surface of the coating, resulting in a freely corroding outer layer and the formation of a beneficial green oxide layer that prevents marine fouling.
In WO94/08840 marine organism growth is inhibited by applying an organic irritant in particulate form to an adhesive layer provided on a substrate surface, the adhesive being in its uncured state (i.e. wet). Copper granules or a copper wire screen may also be applied to the adhesive layer. The organic irritant migrates to the surface of the layer to provide protection against biofouling, possibly in cooperation with the copper if used.
A problem with the aforementioned techniques is that the coating typically relied upon is rigid and suffers from delamination, particularly when the coating is applied to a flexible structure. In addition to delamination, temperature variations and cycling can contribute to a weakening of chemical bonds which secure the active particles within the coating. As most of these techniques involve the active particles being consumed by migration through the coating and/or through attrition of the coating, there is also a need to regularly re-apply and cure fresh coating. This can be a time consuming and cumbersome task.
The use of high temperature thermal spraying to provide a continuous metal coating is often impractical due to the propensity of polymers to erode under the prolonged application of heat that is required to develop such a coating—most polymers have relatively low melting points and this limits the temperatures that can be effectively applied during thermal spraying. Large differences in thermal expansion coefficient between a metal coating and the polymer substrate can also lead to delamination if heat is applied during spraying.
Protection of polymer and polymer-based materials against marine biofouling is a complex problem which current coating methods, including the above, fail to adequately address. Marine equipment made from polymers is often designed and required to be flexible. For example seismic streamers are long cables housed in a polymer sheath, sometimes over 10 kilometers in length, that in use are towed behind a vessel for geophysical exploration.
The streamers contain sensors and electronics to detect acoustic pulses reflected off geologic structures below the ocean floor. The build-up of biomass due to macrofouling of the streamers can be a significant problem since this will increase drag and affect measurements. However, in this context, prevention of marine fouling is problematic for the following reasons.                Adhesion of an antifouling paint to the polymer sheath is generally not practical due to the size and form of the streamer. Typical antifouling paints are also generally not flexible enough to remain adhered to the polymer sheath.        Attaching a cladding layer of copper or other non-polymeric material onto a polymer substrate is limited by large mismatches in physical properties, particularly elastic modulus and strain to fracture. Cladding would restrict the range of motion that may be required of a flexible polymer.        
Against this background, there is a need to provide a method of providing antifouling properties to a polymer surface that does not suffer the drawbacks associated with conventional techniques as described. The invention is believed to have particular utility in reducing or avoiding biofouling, more particularly marine biofouling, of a polymer surface.